deck - The top side of a paddleboard (n); the floor of a surfboat (n)

ding - A small dent or crack on the outside of a paddleboard (n); to put a ding in a board (v)

diver’s flag - A floating flag used to warn boaters about scuba divers (n)

DNR orders - “Do not resuscitate” orders (legal documents intended to prevent CPR) (n)

double - A beach with two guards working that day (n); a beach that currently has two guards (n)
downcurrent in the direction that the current is moving (adj)

drift - Current (n); to move because of the current (v)

drop-off - A sudden change in ocean depth that catches bathers unaware (n)

duty to act - A responsibility to provide care or assistance due to one’s job (n)